ISLAMABAD, Pakistan — Police fired tear gas to disperse supporters of former Prime Minister Nawaz Sharif who demonstrated near the Pakistani capital for a third day on Friday to protest a court order barring him from elected office.

The unrest and brewing political crisis risk distracting Pakistan's shaky government from the fight against al-Qaida and the Taliban and the country's perilous economic situation.

Police confronted about 100 people who piled up rocks and bricks to block a six-lane highway between Islamabad and the nearby city of Rawalpindi. Officers fired rounds of tear gas, forcing the crowd to disperse.

Police then headed toward the airport, but it was not clear if any protesters had gathered there.

The protests began Wednesday after the Supreme Court upheld a ruling that banned Sharif from contesting elections because of a criminal conviction. Although they have been largely peaceful, mobs erected barricades of burning tires and smashed the windows of stores and banks in Rawalpindi, in Punjab province, on Thursday. Police and protesters clashed and four vehicles were torched.

Interior Ministry spokesman Shahedullah Baig said paramilitary troops had been deployed in Punjab on Friday on request from the provincial authorities. He gave no further details.

The court decisions prevent Sharif from challenging President Asif Ali Zardari in the 2013 general elections.

The Supreme Court also removed Sharif's brother as head of the government in Punjab, Pakistan's richest and most populous province on Wednesday. Zardari compounded the blow by dismissing the Punjab government and putting its governor — a Zardari loyalist — in charge.

Zardari accused of orchestrating rulingsSharif, widely considered the country's most popular politician, has accused Zardari of orchestrating the court rulings and called for protests.

He also has urged his supporters to join mass rallies planned for mid-March by Pakistan's lawyers, whose protests for an independent judiciary undermined former President Pervez Musharraf's long rule.

Dozens of judges who were ousted when Musharraf imposed emergency rule in 2007 have returned to the courts under the year-old government led by Zardari's party.

But the government has blocked the return of Iftikhar Mohammed Chaudhry, the former chief justice who questioned a pact that quashed long-standing corruption charges against Zardari and his slain wife, former leader Benazir Bhutto.

Meanwhile, gunmen opened fire on a school van near the northwestern town of Hangu on Friday, killing its driver and injuring three students before abducting six other children, police officer Arshad Khan said. Those abducted were between the ages of nine and 18, he said.

Pakistan's northwest is a violent, lawless region, where criminal gangs and Islamist militants take refuge.